In the semiconductor industry, there is a need to performance test individual integrated circuits, or "chips," while they are still in their parent wafer. In order to conduct performance tests on these chips, electrical contact must be made with bonding pads formed on the integrated circuit so that appropriate electrical stimulus can be applied to the chips and their respective responses can be determined. A device known as a "probe card" is normally used to make contact with the bonding pads of integrated circuits to allow performance testing.
Probe cards consist of an array of resilient conductors or wires terminating in an array of respective probe points. The wires forming the array of probe points are mounted on a printed circuit board, and the probe points are positioned so that they are precisely aligned with the integrated circuit's bonding pads. A different probe card is generally used for each type of integrated circuit since the bonding pad patterns vary with each integrated circuit. During use, an integrated circuit is positioned below the probe array, with the probe points aligned with respective bonding pads. The wafer and probe array are then brought together so that the probe points slightly deflect as they make contact with their respective bonding pads. The electrical stimuli and responses to the stimuli are conducted through the probe card wires to suitable electronic testing devices. The probe card and integrated circuit are then separated, and the probe points are aligned with another integrated circuit on the wafer to repeat the test until all of the integrated circuits on the wafer have been tested.
In order to maximize production efficiency and minimize the possibility of chip damage, it is desirable to inspect the probe cards to verify their required electrical characteristics and probe point alignment accuracy. The inspection should be done prior to the card's first use and at subsequent intervals to check for wear, damage, or other degradation.
Some of the necessary electrical and mechanical inspections can be performed by currently available devices. For example, machines are commercially available to measure the planarization of probe cards. The term "planarization" refers to the degree of alignment of the probe points in the vertical direction so that they occupy a common horizontal plane. A lack of planarization causes some of the probe wires to bend excessively in order to ensure that all of the probe points make contact with the integrated circuit. This excessive bending of probe wires can cause the probe points to excessively scrape the metal bonding pads thus ruining the chip and producing accelerated wear and oxide buildup on the probe points. Conventional probe card planarization measuring devices function by positioning a flat metal plate adjacent and parallel to the probe point array and then bringing the plate and array together in small increments. After each incremental movement, the electrical continuity between each probe point and the metal plate is measured. The position at which a point makes contact with the plate and initiates continuity determines its position along the Z axis relative to the other points. The plate and array are progressively brought together until all of the probe points have made contact with the plate, thus allowing the Z axis positions of all of the probe points to be determined.
An important electrical characteristic of probe cards is the contact resistance of their probe points. The contact resistance of the probe points gradually increases with use as metal oxides and other contamination adhere to the probe points. Contact resistance is thus measured to determine when the probe points must be cleaned. Contact resistance can be measured by connecting an ohm meter in series with the probe wires as they make contact with the plate of a probe card planarization measuring device.
Another electrical parameter of probe cards which can be measured with an ohm meter is the "board leakage". The "board leakage" is caused by contamination of the probe card and is inversely proportional to the resistance between the probe wires of the card when the wires are isolated from each other and from the planarization plate.
One very important parameter of a probe card that must be measured is the alignment of the probe points along the X and Y axes relative to the bonding pads. The X and Y axes alignment of probe points are currently measured optically by positioning the probe array over its corresponding integrated circuit. The probe array is placed in contact with the bonding pads of the integrated circuit, and the probe wires are deflected an appropriate amount. The positions of the probe points relative to respective bonding pads are then examined visually through a microscope.
This conventional approach to inspecting the probe point alignment along the X and Y axes exhibits a number of limitations. First, since the integrated circuit itself is used to assess the probe point alignment, the probe point alignment cannot be inspected until a corresponding integrated circuit or its artwork has been produced. Thus, it is not possible to inspect the probe cards for an integrated circuit until the integrated circuit is in production. Second, the conventional technique is inherently subjective and labor-intensive, with its attendant expense and susceptibility for errors. Although the need exists for automatically measuring probe point alignment along the X and Y axes, a suitable machine has not been developed.
As mentioned above, it is currently not possible to inspect probe point alignment until a corresponding integrated circuit or its artwork has been produced. It is also not possible to manufacture probe cards using conventional techniques until corresponding integrated circuits or metalization artwork have been produced. Probe cards are currently manufactured by mounting a wafer containing a large number of integrated circuits and a sheet of Mylar on the same table. An operator aligns a viewing aperture with each of the bonding pads on an integrated circuit of the wafer, and punches a hole in the Mylar at a location that is spaced a fixed distance from the optical aperture. An array of holes is thus punched in the Mylar sheet at locations corresponding to the positions of the bonding pads of the integrated circuit. Pointed probe wires are then placed through each of the holes in the Mylar and are secured to a printed circuit card by a suitable adhesive. After the adhesive has hardened, the Mylar sheet is removed, leaving the wires at their proper locations. The wires are then sanded to make their points flat and the array planar. An integrated circuit is thus a critical component in the manufacture of the Mylar sheet used to position the probe wires for the corresponding probe card, thereby making it impossible to manufacture probe cards until corresponding integrated circuits or its artwork have been produced.